The present invention relates to an antenna system and, more particularly, to an antenna system for a portable radio telephone or similar portable radio apparatus.
With a portable radio apparatus such as a portable radio telephone, it is desirable from the portability standpoint that when transmission or reception is not effected or is effected at low levels, a regular antenna be received in the casing of the apparatus to reduce the overall dimensions of the apparatus. An implementation for receiving the antenna as mentioned has been proposed in various forms in the past, as follows.
Specifically, the antenna may be bodily removed from the portable radio apparatus to allow the apparatus to be carried in a smaller configuration. In this case, however, brings about a problem that mounting and dismounting the antenna is troublesome, and a problem that the antenna physically separated from the apparatus can easily be lost. Moreover, the radio apparatus is practically unable to transmit or receive a radio signal without the antenna.
The antenna may be provided with a foldable structure to reduce the length of the radio apparatus. A drawback with the foldable antenna scheme is that, in the folded position, the radiation characteristics of the antenna are effected by a shield structure and a printed circuit board which are usually incorporated in a radio apparatus, obstructing adequate transmission and reception. In addition, the antenna exposed to the outside at all times is not desirable for cosmetic reasons.
Another implementation heretofore proposed is such that when the radio equipment is not operated or is operated at low levels, the regular antenna is retracted into the casing of the apparatus and switched to a built-in or internal antenna. This approach, however, is not practicable without resorting to two independent antennas and a switch for switching them over. This, coupled with the fact that a mechanism for interlocking the antenna switching operations is indispensable, results in the need for substantial space, impeding the miniaturization of such a radio apparatus. Further, the extra switch aggravates the high-frequency losses of signals. The radio apparatus with this antenna switching scheme would be complicated in structure and, therefore, expensive.
A telescopic antenna is another scheme for enhancing the portability of a portable radio apparatus. This type of antenna is telescoped into the casing of the apparatus when a small antenna gain suffices. A telescopic antenna, however, lacks sufficient mechanical strength since the diameter thereof is extremely small at the tip. Such an antenna also requires an extra space for its accommodation in the casing of the apparatus, thus obstructing the miniaturization of the apparatus. Further, the radiation characteristics of the antenna are critically degraded when the antenna is telescopically retracted.
Even though the portability is required, it is desirable in a portable radio apparatus that an antenna be provided with a sufficient gain with respect to a service area and be configured to allow an external amplifier, for example, to be connected thereto.